The selected cardiac phase is a decisive criterion for good image quality in cardiac imaging using CT. Therefore, reliably predicting the cardiac cycle length is the central criterion for all prospective cardiac CT modes (sequence, high-pitch spiral). Without high-quality predictions, prospectively-triggered scans cannot be performed reliably and the desired image quality is not obtained.
According to the current prior art, a relatively simple mechanism is used in practice for predicting the time of the next R wave. This is usually the median value from the length of the last three cardiac cycles, with, additionally, an interval being determined within which the next R wave occurs. In the process, the interval is calculated with the aid of the variance of the heart rate. During the CT examination, the dose power of the X-ray tube can be controlled using the calculated interval such that there is maximum dose power available during the respective rest phase used for the reconstruction, but only little or no radiation dose is applied in cycle phases that are less relevant for imaging. As a result, the data in the desired cardiac phase is first of all available in the subsequent reconstruction, but the applied radiation dose overall is kept as low as possible. In most cases this procedure is sufficient and leads to good results.
However, there are patients that have arrhythmic cardiac cycles, for example as a result of ventricular extrasystoles, and so the above-described averaging over a number of cardiac cycles does not lead to a reliable prediction of cycle lengths. However, such arrhythmic cardiac cycles have typical patterns over a number of cycles and these can be identified by correspondingly longer observation, making predictions possible. Accordingly, it is also well-known to identify such patterns using long-term EKGs, evaluate them statistically and then make corresponding predictions.
However, the problem with such predictions on the basis of long-term EKGs lies in the fact that at the time of recording a long-term EKG there are, in principle, different living conditions than during the CT examination, and as a result, physical and mental states, which occur during the CT examination and influence the cardiac rhythm, do not occur in advance and therefore cannot be taken into account. Furthermore, cardiac CT examinations are generally performed under the administration of contrast agents that likewise heavily burden the circulatory system. This situation too can only be included in the measurements in advance with great difficulty.